1. Field of the Invention
The present invention relates to a structure of an integrated inductor, especially to a structure of an integrated inductor that provides a high quality factor Q, a large bandwidth, and good symmetry.
2. Description of Related Art
An on-chip inductor is a kind of integrated inductor structure, which is usually of a spiral shape. FIG. 1 illustrates a conventional asymmetric spiral inductor. The asymmetric spiral inductor 100 includes a spiral-shaped metal segment 110 (in light gray color) and a metal segment 120 (in dark gray color). The metal segment 110 and the metal segment 120 are disposed on different layers in a semiconductor structure; for example the metal segment 110 is on the upper layer and the metal segment 120 is on the lower layer, as shown in FIG. 1. The metal segment 110 and the metal segment 120 are connected via a connecting structure 130, which can be a via structure in a semiconductor manufacturing process. FIG. 2 is a cross section of the asymmetric spiral inductor 100 in FIG. 1. The lowermost layer is a substrate 210 and on top of the substrate 210 is an oxide layer 220. The metal segment 120 is contained in the oxide layer 220 while the metal segment 110 is on top of the oxide layer 220. The connecting structure 130, which is made up of a via array, forms a plurality of via holes on the surface of the oxide layer 220 and connect the metal segment 110 and the metal segment 120. In general, the metal segment 120 is made on an ultra-thick metal (UTM) layer, which is usually made of copper and is the upmost metal layer of the oxide layer 220, whereas the metal segment 110 is made on the re-distribution layer (RDL), which is usually made of aluminum-copper alloy and is on top of the oxide layer 220. Specifically, the oxide layer 220 is a protection layer formed in a passivation process of semiconductor manufacture and is usually made of SiO2 or SiN3.
The number of turns of the metal segment 110 is 3, and can be increased to enhance the inductance of the asymmetric spiral inductor 100. The increase in the number of turns results in an increase in the area of the asymmetric spiral inductor 100, and in an increase in the parasitic series resistance and the parasitic capacitance of the asymmetric spiral inductor 100 as well, which decrease the self-resonant frequency and the quality factor Q. In addition, metal loss and substrate loss are also key factors to the quality factor Q. The metal loss arises from resistance of the metal itself while the substrate loss arises from two situations. One is caused by a time-varying electric displacement between the metal coil of the inductor and the substrate as the inductor is functioning. The time-varying electric displacement causes a displacement current between the metal coil and the substrate that penetrates into the low-impedance substrate and in turn causes energy loss. The magnitude of the displacement current is related to the area of the inductor; the bigger the area, the higher the displacement current. The other is caused by a tune-varying electromagnetic field of the inductor that penetrates through a dielectric layer and causes a magnetically induced eddy current in the substrate, which flows in a direction opposite to the current direction in the inductor and thus causes energy loss.
A center tap of the inductor is hard to decide because of the asymmetric structure of the asymmetric spiral inductor. Moreover, the asymmetric spiral inductor is impractical for being used as a passive component in a differential circuit because positions of the inductive center, the capacitive center and the resistive center are different. FIG. 3 shows a conventional symmetric spiral inductor. The symmetric spiral inductor 300 can be roughly divided into an outer part and an inner part. The metal segment 310 includes the left portion of the outer part and the entire inner part; the metal segment 330 includes the right portion of the outer part. The metal segment 310 and the metal segment 330 belong to the same metal layer in the structure (in dark gray color) and are connected by a bridging metal segment 320 of another metal layer (in light gray color). The center of the inner part is connected to a center tap 340, which is on a layer different from the metal segments 310 and 330 and the bridging metal segment 320. A connecting structure 350, a connecting structure 360 and a connecting structure 370 respectively connects the metal segment 310 and the bridging metal segment 320, the bridging metal segment 320 and the metal segment 330, and the metal segment 310 and the center tap 340. The connecting structures can be implemented by vias. Since the symmetric spiral inductor 300 is symmetric in structure, its center tap 340 is easy to decide. Two inductors are respectively defined by the terminal 342 of the center tap 340 and the terminal 312 of the metal segment 310 as well as by the terminal 342 of the center tap 340 and the terminal 332 of the metal segment 330. Ideally, these two inductors have similar inductance, but a practical analysis of the current path of each inductor renders an unideal consequence. A current from the terminal 332 to the center tap 340 (dashed line) flows sequentially through the right portion of the outer part (i.e., the metal segment 330), the connecting structure 360, the bridging metal segment 320, the connecting structure 350 and the left portion of the inner part; on the other hand, the current from the terminal 312 to the center tap 340 flows through only the left portion of the outer part and the right portion of the inner part. Generally, resistances of different metal layers are not the same and the connecting structure also increases the resistance, which accounts for differences in the inductances of the two inductors. When the two inductors are being used as the inductor 410 and the inductor 420 of the VCO (voltage controlled oscillator) in FIG. 4, asymmetric inductances may cause common mode phenomenon in this differential circuit, which affects the stability of the circuit.
In addition, a metal loss of an inductor operating in a low frequency arises from the series resistance of the metal coil when the current in the metal coil has a uniform distribution. When the inductor operates at a high frequency, the inner metal coil generates a high magnetic field, which induces an eddy current inside the metal coil that causes the skin effect phenomenon. Under the skin effect phenomenon, most current is pushed to the surface of the metal coil by the eddy current, which results in uneven current distribution and in turn degrades the quality factor Q because the current encounters a greater resistance as flowing through a smaller cross section of the metal.